Diode-lasers are efficient devices for converting electrical power into coherent optical power. In this respect, they represent the most efficient class of laser devices. An edge-emitting diode-laser has a diode-laser emitter grown on a single-crystal substrate. The diode-laser emitter emits laser-radiation through an end-facet in an emission direction. For high-power applications, diode-laser bars having a plurality of such diode-laser emitters provide a convenient way to scale optical power. In a diode-laser bar, individual diode-laser emitters are formed in a plurality of epitaxially grown semiconductor layers on a single-crystal substrate. A typical diode-laser bar has a linear array of such diode-laser emitters that generate a corresponding plurality of individual optical beams propagating in the emission direction.
Commercial diode-laser bars are usually “packaged” into assemblies that also include electrical connectors and a base element. The base element includes features for mechanical mounting and provides cooling. In a “conductively cooled package” the base element has sufficient mass to remove waste heat from the diode-laser bar. For higher power operation, the base element is typically water-cooled, for example through a micro-channel arrangement.
The optical beam from an individual diode-laser emitter is divergent in a slow-axis direction and highly-divergent in an orthogonal fast-axis direction. Typically, the optical beams of all the emitters along a diode-laser bar are collimated in the fast-axis direction by a single high-power cylindrical lens, called a “fast-axis collimating lens”. The fast-axis collimating lens is located directly in front of the diode-laser emitters and extends along the width of the bar to capture all of the optical power. Typically, slow-axis collimation is achieved using a single-optical element shaped into a linear array of cylindrical micro-lenses. The pitch of the lens array matches the spacing between diode-laser emitters. The slow-axis collimating element is located to intercept all of the optical power transmitted by the fast-axis collimating lens.
For further power scaling, a plurality of diode-laser bar assemblies are stacked together “vertically” to make a two-dimensional array of diode-laser emitters. Such a diode-laser stack can produce kilowatts (kW) of optical power. Optical output from the entire diode-laser stack is approximately collimated in both the fast-axis and slow-axis directions by above-discussed collimating-lens elements in front of each diode-laser bar. Electrically connecting all the diode-laser bar assemblies in series enables the whole stack to be driven from one current source. Similarly, stacking the diode-laser bar assemblies enables the whole stack to be cooled in parallel by a single source of cooling water.
Diode-laser stacks are utilized in diverse applications, including laser welding, heat treatment of metals (hardening and cladding), medical therapies, and pumping of other solid-state lasers. Many of these applications benefit from tight focusing of the optical output from the stack. For example, diode-laser stacks are an optical pump source for high-power fiber resonators and amplifiers. However, pump-light from a diode-laser stack must be focused into a pump-cladding with a diameter on the order of 100 micro-meters (μm) to 1 millimeter (mm). Ideally, a single-element lens would be used to focus the optical output from a stack to a tight focus. However, single-element lenses fabricated by conventional methods have optical aberrations. Spherical aberration, in particular, degrades the focusing of lenses that have large diameter relative to focal length. Such a large diameter lens is typically required to capture all light from a diode-laser stack. Spherical aberration accordingly limits the power scaling that can be achieved by stacking additional diode-laser bar assemblies.
Corrective phase-plates are a commonly used means for overcoming spherical aberration. Corrective phase-plates are customized by an expensive process that involves precise wave-front characterization of the optical output of a diode-laser stack, then fabrication of a specific phase-plate element. Alternatively, large-aperture aspheric lenses that are comparable in size to a diode-laser stack can be fabricated free of spherical aberration. However, such large aspheric lenses are expensive, becoming a significant fraction of the cost of a complete diode-laser stack light-source. There is need for an improved diode-laser stack producing optical output that can be tightly focused by a simple and inexpensive single-element lens.